The present invention relates to a surface acoustic wave device constructed by input and output transducers formed on a piezoelectric substrate and, in particular, to a surface acoustic filter effectively used in the mobile communications field.
In a surface acoustic wave device used in the field of a mobile communications apparatus, not only a low loss but also an abrupt out-of-band blocking characteristic is required. In a GSM-IF filter standardized in the European countries, for example, a low loss characteristic are broader band are required, and in addition out-of-band blocking characteristic is also required since a number of channels are located close to the broader band.
In order to obtain such characteristic, use has been made of {circle around (1)} a resonator type filter having a plurality of cascade-connected surface acoustic resonator filters, {circle around (2)} a resonator type filter having a plurality of surface acoustic resonator filters connected in a parallel array in an opposite phase relation, and {circle around (3)} an R(resonant)-SPUDT(Single Phase Uni-directional Transducer) type filter in which respective transducers are constructed of SPUDTs and associated transducers are comprised of a normal SPUDT section having a surface wave propagation direction and a counter SPUDT section having a counter surface wave propagation direction, and so on.
In the resonator type filter of {circle around (1)} however, many more connection stages are needed to obtain an abrupt blocking characteristic, thus involving a disadvantage of an insertion loss. Further, in order to obtain a broader band characteristic it is necessary to take a phase matching of the individual resonators, so that it is difficult to mount them.
In the filter of {circle around (2)}, the passband width are broadened through the multiplexing of the frequency characteristic of the respective resonator filters and, in any frequency area, the phase characteristics of the respective resonator filters are so designed as to be made opposite in polarity. If, therefore, the frequency interval of a multiplexing mode of one resonant filter is even slightly displaced from a designed value, a distortion appears in the passband, thus making it difficult to mass produce filters of uniform characteristic. In order to suppress any unwanted high-order area outside the passband, there was the drawback of requiring a high degree of adjustment on the phase characteristic of the two resonators.
In the R-SPUDT type filter, both the passband width and skirt characteristic (falling characteristic of a slop portion adjacent to the passband) depend upon the surface wave reflectivity of the transducer and it is difficult to adjust these characteristics (passband width and skirt characteristic) independently. It was, therefore, difficult to design a filter of a broader band and abrupt blocking characteristic.
It is accordingly the object of the present invention to provide a surface acoustic wave device capable of freely setting the frequency passband width and blocking characteristic without requiring any high degree of designing and adjustment.
A surface acoustic wave device of the present invention is comprised of two parallel-connected surface acoustic filters of a main filter Fm and sub-filter FS. The device is characterized in that, relative to the frequency band constituting a passband of the main filter Fm, the sub-filter Fs has a frequency blocking band (attenuation band) and that, relative to a slope portion outside the passband of the main filter, the sub-filter Fs has a rising area of a frequency-attenuation amount characteristic.
An overall characteristic of the passband of the surface acoustic wave device as a whole is determined by a resultant characteristic of individual characteristic of the main filter Fm and sub-filter Fs. And, in the surface acoustic wave device, the passband width of the device as a whole is determined mainly by the passband width of the main filter Fm. That is, in the passband of the main filter Fm, the characteristic of the sub-filter Fs defined a blocking band and, in this blocking area, the sub-filter Fs is electrically open or in a high impedance state as seen from the main filter Fm. As a result, in the frequency band, the characteristic of the main filter Fm is directly reflected on the overall characteristic of the element as a whole.
In a frequency area outside the passband of the main filter Fm, the sub-filter Fs has a rising area and, in this area, the sub-filter Fs allows an input signal to pass through only at a predetermined level. Through the utilization of the signal passed through the sub-filter Fs it is possible to obtain any resultant characteristic with a signal passed through the main filter Fm.
Stated in more detail, in a predetermined frequency area (slope portion) outside the passband of the main filter Fm, a signal passing through the sub-filter Fs is so set as to have a different phase from a signal passing through the main filter Fm and, more preferably, these signals are so set as to have mutually opposite polarity phase characteristics. By doing so, the signals passing through the main filter Fm and sub-filter Fs are cancelled with respect to each other in a predetermined frequency area and, from an outer appearance of the element as a whole, the passing signal at an unwanted band is blocked. Therefore, the output signal level (unwanted signal level) of the device as a whole in a frequency area outside the passband of the main filter Fm can be made more suppressed than the output signal level of each of the main filter Fm and sub-filter Fs. More preferably, if, in this frequency area, the phases of the signals of the main filter Fm and sub-filter Fs are made to 180xc2x0 differ and the amplitude levels of their passing characteristics are made the same, a resultant output signal of the filter is completely blocked in this area. By setting a mutually cancelled signal area to a slope portion of the frequency-attenuation amount characteristic it is possible to obtain an abrupt blocking characteristic from an overall viewpoint.
Thus, the passband characteristic of the device as a whole can be determined by the passband characteristic alone of the main filter Fm while, on the other hand, the out-of-passband characteristic of the device as a whole has only to properly adjust the phase of the signal of the sub-filter Fs relative to the phase of the signal of the main filter Fm. It is, therefore, possible to freely set the bandwidth and blocking characteristic of the device as a whole by a simple method.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.